Finding critical AoA for an aircraft

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theonlyski@reddit

IIRC, most trainers have a critical AOA of \~17*. I don't know where that's written down though.

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mtconnol@reddit

It is a function of the airfoil design and nothing else. If you know the NACA identifier of the airfoil you can find out the critical AoA.

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primalbluewolf@reddit

It is a function of the airfoil design and nothing else.

So, hypothetically, if we were to operate at a different Reynolds number, the critical AoA would be unchanged?

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mig82au@reddit

How different? IIRC your typical GA aircraft operate well into the turbulent flow regime and critical AoA stays constant until transonic effects show up.

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Well, comparing say a Re of 4 vs 40,000 vs 400,000 vs 4,000,000 - the Cl vs AoA graph for NACA 2412 is available for Re between 50,000 and 1,000,000 on airfoiltools.com for example. The peak Cl and the according critical AoA for that peak Cl both increase, as Re increases.

Hence my point above, that it is not solely and simply a function of the airfoil design and nothing else.

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mtconnol@reddit

Ok, it is a function of the airfoil design, the strong and weak nuclear forces, Planck’s constant, and probably 20 other physical constants. We good?

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indecision_killingme@reddit

Pitch up until she stalls, that’s your critical AOA.

I have yet to see a precise AOA indicator in single pistons, so why do you care?

An indirect indicator is IAS at 1 G.

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Av8torryan@reddit

Critical AOA depends on the wind design and flap setting. It’s not published in any POH. They publish a stall speed for level flight and that’s about it . Just cinder what happens at Crot AOA .

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MockCheckrideDotCom@reddit

17 for the Archer, 16 for most 172s, if I remember correctly.

But as others have said, it's not something that students need to know to be able to be effective with inadvertent stall and spin avoidance.

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Raccoon_Ratatouille@reddit

A better question is why does it matter?

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voretaq7@reddit

The aircraft is a piper pilot (PA-28-181).

Then the critical angle of attack is right around when the stall buzzer goes off (or on really old PA28s when the little red light flashes).

That's really all you and your student need to know - this is about being a pilot, not an aeronautical engineer, and for your practical pilot purposes the PA28 stall warning is an AOA indicator. It's just much less sophisticated than the dedicated AOA indicators you can get now that provide more continuous measurements - the one Piper gives you only knows from "This is fine." and "Oh shit, you're about to stall the goddamn wing!"

(Like others said you can look up the airfoil and get the exact number, but of what practical use is that? It doesn't exactly correspond to deck angle or anything you could conveniently measure within the airplane, and ultimately what matters about critical AOA and exceeding it is "That's when the airflow over the wing breaks down, the wing stalls, and you stop flying.")

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OrganicParamedic6606@reddit

Doesn’t matter. If it’s 14 degrees or 18 degrees, what will the student do differently? Remember your lesson plans are tailored to the student and their needs.

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Prof_Slappopotamus@reddit

It's right where the stall happens.

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FL060@reddit

They aren't published?

You can find your approximate AoA if you have a Flight Path Vector available on your PFD. Just subtract the FPV angle from your pitch angle.

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Junior_Ability7985@reddit

… and add the angle of incidence. Also not publushed.

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primalbluewolf@reddit

Technically speaking? Doesn't matter whether you add it or not.

There's a bunch of different references you could choose to use for measuring AoA against. Sometimes its the aircraft boresight / gun cross. Sometimes its the direction the air data probe points.

Whichever reference you use, the only thing that matters is that everyone you're discussing it with is using the same reference.

Then there's deciding what defines zero AoA - geometrically, such that the gun boresight is aligned with the free stream? Or absolutely, such that the lift coefficient is zero? Both have their merits, but you can't compare graphs from different textbooks or manuals without qualifying which zero they've picked. Most into aeronautics textbooks for pilots neglect to even mention that its a factor, as they don't expect most pilots to need to quantify AoA in the first place.

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DanThePilot_Mann@reddit

What? AOA is definitionally the angle between the chord line of the wing and the relative wind.

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primalbluewolf@reddit

That's geometric AoA, and its not uncommon to see it used. In which case, it really doesn't matter which reference you use.

Things that affect the chord line of the wing that modify your geometric AoA with this definition include LE and TE flap/flaperon deployments, which may not be convenient for a variety of reasons. It also makes it hard to directly compare Cl graphs between wing designs in different flap settings.

Absolute AoA (defined with zero AoA = zero lift) is more useful in design often.

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FL060@reddit

That's why I said approximate. Just trying to get a ballpark.

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EngineerFly@reddit

If you really want to know the critical AoA:

Find out what airfoil the airplane uses. That’s often in the flight manual. Most small single-engine Cessna use NACA 2412.

Then look up its airfoil data, either online or in Abott & von Doenhoff.

Then remember that pilots really don’t care what that number is. They care about sensing that they’re approaching it, not exceeding it, and recovering when they do.

And it’ll be about 15°

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cazzipropri@reddit

Step 1. Find out what airfoil the wing design is based off, e.g., NACA 65(2)-415;

Step 2. Find out the critical angle of attack for that airfoil, e.g., for NACA 65(2)-415 it's 15--17 degrees.

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tempskawt@reddit

Could look up the NACA information on the airfoil design. But yeah, I don’t think POH would have information like that because where would you be measuring AOA? The wing is twisted, so there is some ambiguity there.

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Nnumber@reddit

Came here for this. Might find the NACA in the TCDS.

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Positive-Hat2127@reddit

I fail to see the point of knowing the exact value of critical AoA.

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DillDeer@reddit

I was told 20° generally

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TxAggieMike@reddit

Be careful about going to far down detail rabbit holes.

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BrtFrkwr@reddit

Or in other words, don't overscrew your chicken.

It's unfortunate that we don't have angle of attack indicators as it's much more accurate a way to shoot an approach than published airspeeds.

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primalbluewolf@reddit

Our Six now has an AoA indicator on the G3X. Bringing back a lot of memories from fast jet sims...

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srkjb@reddit

http://airfoiltools.com/

gives you a numerical answer if you can find the airfoil design for your aircraft and its in their database. People will be shocked to learn airofoils actually dont always stall at the same AoA, but thats getting into the weeds too much.

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JustAnotherDude1990@reddit

They dont tell you because you have no way of measuring, and the way you find it is by finding where the stall is. General AoA is like 16-18 degrees or something, but specifics dont really matter here.

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VileInventor@reddit

17-20 degrees. I think I read it in the PHAK or Airplane Flying Handbook

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phliar@reddit

I assume your airplane does not have an AoA indicator. What use is it to a pilot to know the value of the critical AoA?

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rFlyingTower@reddit

This is a copy of the original post body for posterity:

Working on my CFI lesson plans, specifically principles of flight and just realized I have no idea what the critical aoa is for the aircraft I fly or where to find it. I checked the POH and nothing. Anyone know where I could find it? The aircraft is a piper pilot (PA-28-181).

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